The most common call type is a call established between two parties for one-to-one communication. The standard way to set up a two-party call requires explicit control plane signalling that allows the call parties to establish a channel where the audio data can be transferred and to negotiate the communication capabilities; for example, the audio codec and the relative compression rate can be determined in this phase. Afterwards the actual voice communication can start and the audio data can be transmitted by the call parties.
Voice over Internet Protocol (VoIP) enables a speech communication over an IP connection. The Session Initiation Protocol (SIP, RFC 2543) is conventionally used for call establishment in “VoIP” based communication systems.
A mobile communications system refers generally to any telecommunications system which enables communication when users are moving within the service area of the system. A typical mobile communications system is a Public Land Mobile Network (PLMN). Often the mobile communications network is an access network providing a user with wireless access to external networks, hosts, or services offered by specific service providers.
Professional mobile radio or private mobile radio (PMR) systems are dedicated radio systems developed primarily for professional and governmental users, such as the police, military forces, oil plants, etc. PMR services are offered via dedicated PMR networks built with dedicated PMR technologies. This market is divided between several technologies—analog, digital, conventional and trunked—none of which has a dominating role. TETRA (Terrestrial Trunked Radio) is a standard defined by ETSI (European Telecommunications Standards Institute) for digital PMR systems.
One special feature offered by the PMR systems is group communication. The term “group”, as used herein, refers to any logical group of three or more users intended to participate in the same group communication, e.g. a call. Group communication with a push-to-talk feature is one of the essential features of any PMR network. Generally, in group voice communication with a “push-to-talk, release-to-listen” feature, a group call is based on the use of a pressel (PTT, push-to-talk switch) in a telephone as a switch: by pressing a PTT the user indicates his desire to speak, and the user equipment sends a service request to the network. The network either rejects the request or allocates the requested resources on the basis of predetermined criteria, such as the availability of resources, priority of the requesting user, etc. At the same time, a connection is established also to all other active users in the specific subscriber group. After the voice connection has been established, the requesting user can talk and the other users can listen on the channel. When the user releases the PTT, the user equipment signals a release message to the network, and the resources are released. Thus, the resources are reserved only for the actual speech transaction or speech item, instead of reserving the resources for a “call”. One interesting advantage of the push-to-talk communication, or more generally speech-item-by-speech-item communication, is a short call setup time, which also makes such speech communication attractive to several other types of users. U.S. Pat. No. 6,141,347 discloses a wireless communications system which uses multicast addressing and decentralized processing in group calls.
A problem with such item-by-item communication is that a strict discipline or protocol is required from the parties in the speech communication, or other type of real-time data communication. Further, especially in group communication, it is difficult to know which the parties of the communication are at each specific moment, and therefore the communication must include spoken questions and acknowledgements.